1. Technical Field
The present invention relates generally to bipolar transistors and more specifically to double polysilicon bipolar transistors.
2. Background Art
Bipolar transistors were originally developed many years ago but were generally superceded by CMOS transistors, which consume less power and are less difficult to manufacture, and thus less expensive. Although a great deal of effort has been spent over the years improving bipolar transistors, they have not improved to the point where they are as popular as CMOS transistors.
Recently, demand for bipolar transistors has increased significantly because these transistors are capable of operating at higher speeds and driving more current than CMOS transistors. These characteristics are important for high-speed, high frequency communication networks such as those required by cell phones and computers.
Most bipolar transistors in use today are the so-called “double poly” bipolar transistors, which use a two polysilicon structures; one for an emitter structure and a second for a base structure.
The conventional bipolar transistor is difficult to manufacture and cost is a major problem. The transistor is manufactured by implanting a substrate with a dopant to provide a buried collector. An additional layer of silicon covers the implanted substrate. Insulating dividers called shallow-trench isolation (STI) are formed in the silicon substrate. The STI's define an intrinsic base region over a buried collector and a collector tap connected to the buried collector between two of the STI's and separated from the intrinsic base region.
Subsequently, a first layer of polysilicon is deposited over the upper silicon and is processed to form a base polysilicon structure in contact with the intrinsic base region. One portion of the base polysilicon structure is formed with an opening for an emitter polysilicon structure to be formed within.
A first insulating layer is deposited over the base polysilicon structure and is removed in the opening of the base polysilicon structure over the intrinsic base region by etching down to the substrate. The process inherently produces a rough surface on the substrate.
To get higher performance, silicon germanium is generally grown over the insulating layer and on the rough surface of the substrate. The rough surface causes a major problem because the silicon germanium growth is irregular and its thickness is not constant as a result of the roughness. This leads to performance problems with the device and differences from device to device.
The silicon germanium is processed to surround and cover the base polysilicon structure and a second insulating layer is grown over the silicon germanium. The second insulating layer is then removed over the silicon germanium on the intrinsic base region.
A second layer of polysilicon is deposited and processed to form an emitter polysilicon structure, which is encircled by and overlaps the base polysilicon structure. The overlap is necessary to provide room for an emitter contact, but it causes another major problem with unwanted capacitance between the emitter and base polysilicon structures. This capacitance slows down the operation of the bipolar transistor.
A spacer layer is deposited over the emitter polysilicon structure and is processed to form spacers around the emitter polysilicon structure. An interlayer dielectric layer is then deposited over the emitter and base polysilicon structures and the second insulating layer.
Finally, contacts are formed for the collector, the base, and the emitter.
The collector contact extends through the inner layer dielectric and the second insulating layer to the collector tap connected to the buried collector.
The base contact extends through the inner layer dielectric, the first insulating layer, the silicon germanium, and the second insulating layer, and into the base polysilicon structure.
The emitter contact extends through the inner layer dielectric into the emitter polysilicon structure. As previously mentioned, the emitter polysilicon structure overlaps the base polysilicon structure because it is necessary to provide room for the emitter contact. Since it is desirable to make the overlap as small as possible, it is desirable to have the emitter polysilicon structure as small as possible. However, this means that variations in the size of the emitter contact will lead to a further major problem referred to as emitter shadowing effect where the size of the emitter contact cause different current gains through the emitter polysilicon structure because the emitter dopant will be implanted at different depths. This effect causes performance variations in bipolar transistor current driving capability.
Despite many years of research and development, solutions to these problems have been long sought but have long eluded those skilled in the art.